Amblyopia, or lazy eye, refers to a unilateral or bilateral decrease of vision, in one or both eyes, caused by abnormal vision development in childhood or infancy. It is a common vision problem in children and is the leading cause of decreased vision among children. Most vision loss is preventable or reversible with the right kind of intervention.
The history should address the following:
The physical examination should include the following:
A full eye examination must be performed to rule out ocular pathology.
The most important causes of amblyopia are as follows[1, 2] :
See Clinical Presentation for more detail.
Imaging studies that may be helpful when the ocular examination is normal and suspicion of an organic cause exists include the following:
Other tests that may be helpful in diagnosis include the following:
See Workup for more detail.
First, rule out an organic cause and treat any obstacle to vision, such as cataract or ptosis, if clinically significant. Next, treat anisometropia and refractive errors. Then, initiate occlusion therapy (to force the use of the amblyopic eye) or penalization therapy.
Occlusion therapy
The following are general guidelines for occlusion therapy:
Other treatment
Additional treatment options include the following:
Treatment of strabismus generally occurs last. The endpoint of strabismic amblyopia is freely alternating fixation with equal vision. Surgery generally is performed after this endpoint has been reached.
See Treatment and Medication for more detail.
Amblyopia refers to a decrease of vision, either unilaterally or bilaterally, for which no cause can be found by physical examination of the eye. The term functional amblyopia often is used to describe amblyopia, which is potentially reversible by occlusion therapy. Organic amblyopia refers to irreversible amblyopia caused by ocular pathology that will limit vision improvement.[1, 2, 4]
Most vision loss from amblyopia is preventable or reversible with the right kind of intervention. The recovery of vision depends on how mature the visual connections are, the length of deprivation, and at what age the therapy is begun. It is important to rule out any organic cause of decreased vision because some diseases may not be easily detectable on routine examination.
Although many types of amblyopia exist, it is believed that their basic mechanisms are the same even though each factor may contribute different amounts to each specific type of amblyopia. In general, amblyopia is believed to result from disuse from inadequate foveal or peripheral retinal stimulation and/or abnormal binocular interaction that causes different visual input from the foveae.[5]
Three critical periods of human visual acuity development have been determined.[6, 7] During these time periods, vision can be affected by the various mechanisms to cause or reverse amblyopia. These periods are as follows:
Whether different visual functions (eg, contrast sensitivity, stereopsis) have different critical periods is not known. In the future, determination of these time frames may help modify treatment of amblyopia.
Prevalence of amblyopia is difficult to assess and varies in the literature, ranging from 1-3.5% in healthy children to 4-5.3% in children with ophthalmic problems. Most data show that about 2% of the general population has amblyopia.
Amblyopia was shown in the Visual Acuity Impairment Survey sponsored by the National Eye Institute (NEI) to be the leading cause of monocular vision loss in adults aged 20-70 years or older. Prevalence of amblyopia has not changed much over the years.
Amblyopia is an important socioeconomic problem. Studies have shown that it is the number one cause of monocular vision loss in adults. Furthermore, persons with amblyopia have a higher risk of becoming blind because of potential loss to the sound eye from other causes.
No racial preference is known.
No gender preference is known.
Amblyopia occurs during the critical periods of visual development. An increased risk exists in those children who are developmentally delayed, were premature, and/or have a positive family history.
After 1 year, about 73% of patients show success after their first trial of occlusion therapy. Studies have shown that the number of patients who retain their level of visual acuity decreases over time to 53% after 3 years.
Risk factors for failure in amblyopia treatment include the following:
A study by Mirabella et al determined that even with successful treatment of an amblyopic eye, perception of images in real-world scenes remains altered in patients with a history of amblyopia.[33]
Parents need to be educated about the importance of treatment and compliance as well as the visual implications because the treatment of amblyopia often lies in the hands of the parents.
For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article How to Instill Your Eyedrops.
Elicit any previous history of patching or eye drops as well as past compliance with these therapies. Elicit any previous history of glasses wear.
Document previous ocular surgery or disease.
In addition to the routine information, obtaining a family history of strabismus or other ocular problems is important because the presence of these ocular problems may predispose a child to amblyopia.
Diagnosis of amblyopia usually requires a 2-line difference of visual acuity between the eyes; however, this definition is somewhat arbitrary and a smaller difference is common.
A common characteristic of amblyopic eyes is difficulty in distinguishing optotypes that are close together. Visual acuity often is better when the patient is presented with single letters rather than a line of letters.
Diagnosis is not an issue in children old enough to read or with use of the HOTV test.
If the child protests with covering of the sound eye, amblyopia can be diagnosed if it is dense.
Fixation preference may be assessed, especially when strabismus is present.
Induced tropia test may be performed by holding a 10-prism diopter before one eye in cases of an orthophoria or a microtropia.
In infants who cross-fixate, pay attention to when the fixation switch occurs; if it occurs near primary position, then visual acuity is equal in both eyes.
Caution should be used when obtaining Teller acuity in children, as grating acuity may be less reduced than Snellen acuity, especially in strabismic amblyopia.
Strabismic and anisometropic amblyopic eyes have marked losses of threshold contrast sensitivity, especially at higher spatial frequencies; this loss increases with the severity of amblyopia.
Patients with strabismic amblyopia may have better visual acuity or less of a decline of visual acuity when tested with neutral density filters compared to the normal eye. This was not found to be true in patients with anisometropic amblyopia or organic disease.
Amblyopia usually is associated with changes in binocular function or stereopsis.
Some patients with amblyopia may consistently fixate with a nonfoveal area of the retina under monocular use of the amblyopic eye, the mechanism of which is unknown. This can be diagnosed by holding a fixation light in the midline in front of the patient and asking them to fixate on it while the normal eye is covered. The reflection of the light will not be centered.
Cycloplegic refraction must be performed in all patients, using retinoscopy to obtain an objective refraction after full cycloplegia. In most cases, the more hyperopic eye or the eye with more astigmatism will be the amblyopic eye. If this is not true, one needs to investigate further for ocular pathology or to confirm the refraction.
Perform a full eye examination to rule out ocular pathology.
Preschool vision screening techniques have included the SureSight Autorefractor and the Randot Preschool Stereoacuity Test. One study has found the Pediatric Vision Scanner (PVS), a laser scanner capable of directly detecting amblyopia or strabismus in preschool children, to be more accurate than either of these, achieving a sensitivity of 98% and a specificity of 88%.[8]
Many causes of amblyopia exist; the most important causes are as follows:[1, 2]
Inhibition of the fovea occurs to eliminate the abnormal binocular interaction caused by one defocused image and one focused image.
This type of amblyopia is more common in patients with anisohypermetropia than anisomyopia. Small amounts of hyperopic anisometropia, such as 1-2 diopters, can induce amblyopia. In myopia, mild myopic anisometropia up to -3.00 diopters usually does not cause amblyopia.
Hypermetropic anisometropia of 1.50 diopters or greater has been shown to be a long-term risk factor for deterioration of visual acuity after occlusion therapy.
The patient favors fixation strongly with one eye and does not alternate fixation. This leads to inhibition of visual input to the retinocortical pathways.
Incidence of amblyopia is greater in esotropic patients than in exotropic patients.
These patients have strabismus associated with anisometropia.
Amblyopia results from disuse or understimulation of the retina. This condition may be unilateral or bilateral. Examples include cataract, corneal opacities, ptosis, and surgical lid closure.[9]
Structural abnormalities of the retina or the optic nerve may be present. Functional amblyopia may be superimposed on the organic visual loss.
The main complication of not treating amblyopia is long-term irreversible vision loss. Most cases of amblyopia are reversible if detected and treated early, so this vision loss is preventable.
If suspicion exists of an organic cause for decreased vision and the ocular examination is normal, then further investigations into retinal or optic nerve causes should be initiated. Studies to perform include imaging of the visual system through CT scan, MRI, and fluorescein angiography to assess the retina.
Although differences in the electrophysiologic responses of normal eyes versus amblyopic eyes have been reported, these techniques remain investigational and the differences are controversial.
Differences have been found using B-scans between the foveal structure of amblyopes compared to those without amblyopia. The differences found include an increase in the foveal thickness of amblyopic eyes along with a reduced foveal pit in the horizontal, but not vertical, meridian.[3]
Histologic studies of the lateral geniculate nucleus in kittens with deprivation amblyopia have shown that cells receiving input from the deprived eye were shrunken and atrophied, while cells receiving input from the nondeprived eye were expanded.
The clinician must first rule out an organic cause and treat any obstacle to vision (eg, cataract, occlusion of the eye from other etiologies).
Remove cataracts in the first 2 months of life, and aphakic correction must occur quickly.
Treatment of anisometropia and refractive errors must occur next.[10, 11, 12, 13] The amblyopic eye must have the most accurate optical correction possible. This should occur prior to any occlusion therapy because vision may improve with spectacles alone. This improvement is frequently seen in patients with unilateral refractive amblyopia.[14]
Full cycloplegic refraction should be given to patients with accommodative esotropia and amblyopia. In other patients, a prescription less than the full plus measurement that was refracted may be prescribed given that the decrease in plus is symmetric between the two eyes. Because accommodative amplitude is believed to be decreased in amblyopic eyes, one needs to be cautious about cutting back too much on the amount of plus. Refractive correction alone has been shown to improve amblyopia in up to one-fourth of patients in a nationwide trial.[15]
Patients with bilateral refractive amblyopia do well with spectacle correction alone, with most children aged 3-10 years achieving 20/25 or better within a year.[16, 17]
The next step is forcing the use of the amblyopic eye by occlusion therapy. Occlusion therapy has been the mainstay of treatment since the 18th century. The following are general guidelines for occlusion therapy:
There is some evidence that having children wear an eye patch for 6 hours daily rather than 2 hours can yield greater improvement in visual acuity at 10 weeks.[22] The significantly greater improvement in visual acuity noted with the more intense patching protocol suggests that this strategy is worth considering in children with residual amblyopia. The results have the following implications:
In the past, penalization therapy was reserved for children who would not wear a patch or in whom compliance was an issue. The Amblyopia Treatment Studies, however, have demonstrated that atropine penalization in patients with moderate amblyopia (defined by the study as visual acuity better than 20/100) is as effective as patching. The Amblyopia Treatment Studies were performed in children aged 3-7 years.[23, 24, 25, 26]
The Amblyopia Treatment Studies have also demonstrated that weekend use of atropine provided an improvement in visual acuity similar to that of daily use of atropine when treating moderate amblyopia in children aged 3-7 years.
Atropine drops or ointment is instilled in the nonamblyopic eye. This therapy is sometimes used in conjunction with patching or occlusion of the glasses (eg, adhesive tape, nail polish) by individual practitioners. In the Amblyopia Treatment Studies that evaluated patching versus atropine penalization, atropine penalization and patching were used in conjunction with 1 hour of near visual activities.
This technique may also be used for maintenance therapy, which is useful, especially in patients with mild amblyopia.
Other options include optical blurring through contact lenses or elevated bifocal segments.[27]
In a prospective cohort study of 105 children who had previously participated in a randomized trial comparing atropine and patching for moderate amblyopia, researchers observed, after controlling for baseline refractive error, a decrease in amblyopic eye spherical equivalent refractive error to less hyperopia. This shift toward emmetropia was associated with ocular alignment, supporting the idea that better motor and sensory fusion promote emmetropization.[28]
The desired endpoint of therapy is spontaneous alternation of fixation or equal visual acuity in both eyes.[29] When visual acuity is stable, patching may be decreased slowly, depending on the child's tendency for the amblyopia to recur. There is no established standard for tapering the patching treatment. Because amblyopia recurs in a large number of patients (see Prognosis), maintenance therapy or tapering of therapy should be strongly considered. This tapering is controversial, so individual physicians vary in their approaches.
Even though there have been many advances in amblyopia treatment, tailoring the treatment with individual treatment plans is still difficult because the dose/effect response from the amount of occlusion is not fully understood.[14]
Treatment of strabismus generally occurs last. The endpoint of strabismic amblyopia is freely alternating fixation or equal vision. Surgery generally is performed when the endpoint has been reached.
Surgical therapy for strabismus generally should occur after amblyopia is reversed. Disadvantages to surgical therapy prior to correction of amblyopia include difficulty in telling if amblyopia is present because there is no longer a strabismus to assess fixation preference in nonverbal patients and higher potential to being lost to follow-up, as the child cosmetically looks better. The improved cosmesis gives the parents a false sense of security about the vision improving.
Close supervision during occlusion therapy is necessary to make sure children do not peek. Various methods of preventing children from removing patches have been considered, from a reward system for older children to arm splints and mittens for infants.
Studies have shown these programs to be technically easy and that they help reduce cost as well as incidence of amblyopia because of early treatment and detection. Current programs include use of the photorefractor and school vision screening programs.
Longmuir et al reported the results of a 9-year, volunteer photoscreening program.[32] From 2000-2009, 147,809 children underwent photoscreening to detect amblyopic risk factors; 6247 children (4.2%) were referred to local eye care professionals. Of the children referred, 24.3% were evaluated by local ophthalmologists and 76.7% were seen by local optometrists. The follow-up rate ranged from 36.1-89.5%, with an overall program follow-up rate after the addition of a follow-up coordinator of 81.3%. Including the overall operational budget, the cost of screening was reduced to $9 per child. Although the Medical Technology and Innovations (MTI) photoscreener used in this program is no longer manufactured, this report does illustrate that cost-effective screening can be done using a volunteer system and demonstrates the problem of successful screening that is not followed with a visit to an eye care professional.
The addition of a part-time follow-up coordinator to the photoscreening program produced 89.5% follow-up rate when screening 147 809 children for amblyopia risk factors over a 9-year period.
Young patients who have trauma to their eyes often are at risk for occlusion amblyopia. Possible reasons include lid edema, hyphema, occlusive dressing, vitreous hemorrhage, corneal opacity or irregularity, and traumatic cataract.
This amblyopia often is superimposed on a visual deficit caused by any structural abnormality and needs to be taken into account when treating these children.
Vision needs to be monitored closely in children after ocular trauma, especially in those aged up to 6 years and in nonverbal children. Occlusive therapy needs to be instituted if there is any suggestion of decreased vision in the injured eye.
Outpatient follow-up care needs to continue beyond the primary completion of amblyopia treatment because visual deterioration occurs in many children. In a multicenter study conducted as part of the Amblyopia Treatment Studies, one fourth of patients experienced recurrence of amblyopia within the first year after treatment, with the risk of recurrence greater if the treatment was stopped abruptly rather than tapered. One study by Levartovsky et al showed deterioration in 75% of children with anisometropia of 1.75 diopters or more after occlusion therapy.[31] Recidivism can occur, even several years after the initial treatment period, and is as high as 53% after 3 years.
Pharmacologic treatment with levodopa has been investigated and has showed transient improvement of vision in amblyopic eyes. However, the exact role of such pharmacologic agents has not been determined. The Pediatric Eye Disease Investigator Group demonstrated that levodopa is not as effective as an adjunctive treatment in amblyopia. Levodopa currently is not being used clinically.[30]
Atropine penalization (with either ointment or drops) is an alternative method of blurring vision in the sound eye of patients who refuse patching. It may be applied once a day to patients in the preferred eye only.
Clinical Context: A topically applied muscarinic antagonist, which blocks the action of acetylcholine. This results in paralysis of the iris sphincter and resultant pupillary dilation. Paralysis of the ciliary muscles also occurs, which inhibits accommodation and relieves pain in iridocyclitis. The medication is dispensed in a topical formulation, either an ointment or a solution.
These agents are used to blur vision in one eye to treat amblyopia in the contralateral eye.